1================== 2The SMBus Protocol 3================== 4 5The following is a summary of the SMBus protocol. It applies to 6all revisions of the protocol (1.0, 1.1, and 2.0). 7Certain protocol features which are not supported by 8this package are briefly described at the end of this document. 9 10Some adapters understand only the SMBus (System Management Bus) protocol, 11which is a subset from the I2C protocol. Fortunately, many devices use 12only the same subset, which makes it possible to put them on an SMBus. 13 14If you write a driver for some I2C device, please try to use the SMBus 15commands if at all possible (if the device uses only that subset of the 16I2C protocol). This makes it possible to use the device driver on both 17SMBus adapters and I2C adapters (the SMBus command set is automatically 18translated to I2C on I2C adapters, but plain I2C commands can not be 19handled at all on most pure SMBus adapters). 20 21Below is a list of SMBus protocol operations, and the functions executing 22them. Note that the names used in the SMBus protocol specifications usually 23don't match these function names. For some of the operations which pass a 24single data byte, the functions using SMBus protocol operation names execute 25a different protocol operation entirely. 26 27Each transaction type corresponds to a functionality flag. Before calling a 28transaction function, a device driver should always check (just once) for 29the corresponding functionality flag to ensure that the underlying I2C 30adapter supports the transaction in question. See :doc:`functionality` for 31the details. 32 33 34Key to symbols 35============== 36 37=============== ============================================================= 38S Start condition 39P Stop condition 40Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0. 41A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) bit 42Addr (7 bits) I2C 7 bit address. Note that this can be expanded as usual to 43 get a 10 bit I2C address. 44Comm (8 bits) Command byte, a data byte which often selects a register on 45 the device. 46Data (8 bits) A plain data byte. Sometimes, I write DataLow, DataHigh 47 for 16 bit data. 48Count (8 bits) A data byte containing the length of a block operation. 49 50[..] Data sent by I2C device, as opposed to data sent by the host 51 adapter. 52=============== ============================================================= 53 54 55SMBus Quick Command 56=================== 57 58This sends a single bit to the device, at the place of the Rd/Wr bit:: 59 60 S Addr Rd/Wr [A] P 61 62Functionality flag: I2C_FUNC_SMBUS_QUICK 63 64 65SMBus Receive Byte 66================== 67 68Implemented by i2c_smbus_read_byte() 69 70This reads a single byte from a device, without specifying a device 71register. Some devices are so simple that this interface is enough; for 72others, it is a shorthand if you want to read the same register as in 73the previous SMBus command:: 74 75 S Addr Rd [A] [Data] NA P 76 77Functionality flag: I2C_FUNC_SMBUS_READ_BYTE 78 79 80SMBus Send Byte 81=============== 82 83Implemented by i2c_smbus_write_byte() 84 85This operation is the reverse of Receive Byte: it sends a single byte 86to a device. See Receive Byte for more information. 87 88:: 89 90 S Addr Wr [A] Data [A] P 91 92Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE 93 94 95SMBus Read Byte 96=============== 97 98Implemented by i2c_smbus_read_byte_data() 99 100This reads a single byte from a device, from a designated register. 101The register is specified through the Comm byte:: 102 103 S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P 104 105Functionality flag: I2C_FUNC_SMBUS_READ_BYTE_DATA 106 107 108SMBus Read Word 109=============== 110 111Implemented by i2c_smbus_read_word_data() 112 113This operation is very like Read Byte; again, data is read from a 114device, from a designated register that is specified through the Comm 115byte. But this time, the data is a complete word (16 bits):: 116 117 S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P 118 119Functionality flag: I2C_FUNC_SMBUS_READ_WORD_DATA 120 121Note the convenience function i2c_smbus_read_word_swapped() is 122available for reads where the two data bytes are the other way 123around (not SMBus compliant, but very popular.) 124 125 126SMBus Write Byte 127================ 128 129Implemented by i2c_smbus_write_byte_data() 130 131This writes a single byte to a device, to a designated register. The 132register is specified through the Comm byte. This is the opposite of 133the Read Byte operation. 134 135:: 136 137 S Addr Wr [A] Comm [A] Data [A] P 138 139Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE_DATA 140 141 142SMBus Write Word 143================ 144 145Implemented by i2c_smbus_write_word_data() 146 147This is the opposite of the Read Word operation. 16 bits 148of data are written to a device, to the designated register that is 149specified through the Comm byte:: 150 151 S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P 152 153Functionality flag: I2C_FUNC_SMBUS_WRITE_WORD_DATA 154 155Note the convenience function i2c_smbus_write_word_swapped() is 156available for writes where the two data bytes are the other way 157around (not SMBus compliant, but very popular.) 158 159 160SMBus Process Call 161================== 162 163This command selects a device register (through the Comm byte), sends 16416 bits of data to it, and reads 16 bits of data in return:: 165 166 S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] 167 S Addr Rd [A] [DataLow] A [DataHigh] NA P 168 169Functionality flag: I2C_FUNC_SMBUS_PROC_CALL 170 171 172SMBus Block Read 173================ 174 175Implemented by i2c_smbus_read_block_data() 176 177This command reads a block of up to 32 bytes from a device, from a 178designated register that is specified through the Comm byte. The amount 179of data is specified by the device in the Count byte. 180 181:: 182 183 S Addr Wr [A] Comm [A] 184 S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P 185 186Functionality flag: I2C_FUNC_SMBUS_READ_BLOCK_DATA 187 188 189SMBus Block Write 190================= 191 192Implemented by i2c_smbus_write_block_data() 193 194The opposite of the Block Read command, this writes up to 32 bytes to 195a device, to a designated register that is specified through the 196Comm byte. The amount of data is specified in the Count byte. 197 198:: 199 200 S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P 201 202Functionality flag: I2C_FUNC_SMBUS_WRITE_BLOCK_DATA 203 204 205SMBus Block Write - Block Read Process Call 206=========================================== 207 208SMBus Block Write - Block Read Process Call was introduced in 209Revision 2.0 of the specification. 210 211This command selects a device register (through the Comm byte), sends 2121 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return:: 213 214 S Addr Wr [A] Comm [A] Count [A] Data [A] ... 215 S Addr Rd [A] [Count] A [Data] ... A P 216 217Functionality flag: I2C_FUNC_SMBUS_BLOCK_PROC_CALL 218 219 220SMBus Host Notify 221================= 222 223This command is sent from a SMBus device acting as a master to the 224SMBus host acting as a slave. 225It is the same form as Write Word, with the command code replaced by the 226alerting device's address. 227 228:: 229 230 [S] [HostAddr] [Wr] A [DevAddr] A [DataLow] A [DataHigh] A [P] 231 232This is implemented in the following way in the Linux kernel: 233 234* I2C bus drivers which support SMBus Host Notify should report 235 I2C_FUNC_SMBUS_HOST_NOTIFY. 236* I2C bus drivers trigger SMBus Host Notify by a call to 237 i2c_handle_smbus_host_notify(). 238* I2C drivers for devices which can trigger SMBus Host Notify will have 239 client->irq assigned to a Host Notify IRQ if noone else specified an other. 240 241There is currently no way to retrieve the data parameter from the client. 242 243 244Packet Error Checking (PEC) 245=========================== 246 247Packet Error Checking was introduced in Revision 1.1 of the specification. 248 249PEC adds a CRC-8 error-checking byte to transfers using it, immediately 250before the terminating STOP. 251 252 253Address Resolution Protocol (ARP) 254================================= 255 256The Address Resolution Protocol was introduced in Revision 2.0 of 257the specification. It is a higher-layer protocol which uses the 258messages above. 259 260ARP adds device enumeration and dynamic address assignment to 261the protocol. All ARP communications use slave address 0x61 and 262require PEC checksums. 263 264 265SMBus Alert 266=========== 267 268SMBus Alert was introduced in Revision 1.0 of the specification. 269 270The SMBus alert protocol allows several SMBus slave devices to share a 271single interrupt pin on the SMBus master, while still allowing the master 272to know which slave triggered the interrupt. 273 274This is implemented the following way in the Linux kernel: 275 276* I2C bus drivers which support SMBus alert should call 277 i2c_new_smbus_alert_device() to install SMBus alert support. 278* I2C drivers for devices which can trigger SMBus alerts should implement 279 the optional alert() callback. 280 281 282I2C Block Transactions 283====================== 284 285The following I2C block transactions are similar to the SMBus Block Read 286and Write operations, except these do not have a Count byte. They are 287supported by the SMBus layer and are described here for completeness, but 288they are *NOT* defined by the SMBus specification. 289 290I2C block transactions do not limit the number of bytes transferred 291but the SMBus layer places a limit of 32 bytes. 292 293 294I2C Block Read 295============== 296 297Implemented by i2c_smbus_read_i2c_block_data() 298 299This command reads a block of bytes from a device, from a 300designated register that is specified through the Comm byte:: 301 302 S Addr Wr [A] Comm [A] 303 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P 304 305Functionality flag: I2C_FUNC_SMBUS_READ_I2C_BLOCK 306 307 308I2C Block Write 309=============== 310 311Implemented by i2c_smbus_write_i2c_block_data() 312 313The opposite of the Block Read command, this writes bytes to 314a device, to a designated register that is specified through the 315Comm byte. Note that command lengths of 0, 2, or more bytes are 316supported as they are indistinguishable from data. 317 318:: 319 320 S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P 321 322Functionality flag: I2C_FUNC_SMBUS_WRITE_I2C_BLOCK 323